1. Field of Use
This invention relates generally to multiplexers and demultiplexers for optical signals. More particularly, this invention relates to multiplexers and demultiplexers that combine and divide light signals on the basis of their wavelength components.
2. Description of the Prior Art
The efficiency of an optical transmission system is greatly limited by the insertion losses of couplers and of more complex components such as multiplexers and demultiplexers (collectively referred to herein as multiplexers unless otherwise noted). The insertion loss of a particular optical component is, of course, approximately the sum of the insertion losses of the elements that make up the optical component. The insertion loss attendant state-of-the-art wavelength division multiplexers is high, thus decreasing the efficiency of an otherwise generally efficient optical system. The individual losses that make up the total insertion loss of a wavelength division multiplexer are represented by the following loss equation: l.sub.t =l.sub.d +l.sub.g +l.sub.a +1.sub.f where l.sub.d is the dispersion loss, l.sub.g is the grating reflection loss, l.sub.a is the aberration loss, and l.sub.f is the Fresnel reflection loss. In state-of-the-art multiplexers l.sub.t easily can greatly exceed 3 dB.
Each of the above individual losses can be identified with certain components or characteristics of a wavelength division multiplexer. Dispersion-broadening losses, l.sub.d, are those losses due to broadening of the angular spectrum (beam spread) within the multiplexer. These losses are determined by the linewidth of the light source, wavelength separation between channels, and the relative positioning of the light sources and detectors about the optical axis of the multiplexer. Losses due to angular dispersion-broadening result because dispersion of the incoming light signal within the multiplexer causes the dispersed beam to have a larger spot size than the fiber core; thereby a portion of the light is not channeled into the output fiber. Grating losses, l.sub.g, are the result of imperfections in the dispersion grating. Aberration losses, l.sub.a, primarily are due to off-axial and chromatic aberration. Fresnel losses, l.sub.f, primarily are due to the light signal passing through the glass-air interface on both sides of the focusing lens.
Furthermore, state-of-the-art wavelength division multiplexers cannot handle multiple wavelength ranges over a broad spectrum. State-of-the-art multiplexers are limited by the physics of single surface relief grating.
State-of-the-art wavelength division multiplexers have not achieved high efficiency. The losses in nearly all of these categories are high. As a consequence, the efficiency of the optical transmission system into which they are placed is drastically reduced. The need for a high efficiency broad-band wavelength division multiplexer is apparent.